[VOIPSEC] RTP or SRTP inside UDP - how understand?

[Simon Horne]

The only way I know of is to detect the key exchange in the signalling and 
disable the RTP transcoding.

You can tell (and it is not definitive) if the payload is encrypted by 
examining the payload length and seeing if it is not exactly the same as 
what would be expected for that particular codec.. Usually the output 
length from the cipher (due to the cipher key length) does not match 
exactly the normal unencrypted payload size. It may only be a couple of 
bytes but it is detectable. However to the intermediary devices (proxies, 
gateways etc) it is handled exactly the same as if it was RTP so legacy 
devices should be able to handle it (except if transcoding of course). SRTP 
on the other hand may require intermediary devices to handle it and the 
packets carry a flag saying "I'm encrypted" which makes it much easier to 
detect.

Simon

At 03:31 AM 28/03/2006, Sergey Vointsev wrote:
> > The more pressing question would be: if you're receiving an SRTP stream,
> > why don't you know about it?
>
>Of course I know.
>But I heard somewhere, that if some gateway in the net receives
>something, that it considers to be RTP packet with some known codec
>used, it can transcode it to some other codec. Am I misinformed? (yes,
>I'm novice to VoIP :)
>So actually I want to know how can we tell such devices "payload is
>encrypted, don't touch it!".
>
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